Method for joining workpiece layers and connecting element and joining device

ABSTRACT

A method for joining at least two workpiece layers ( 1, 2 ) by introducing a connecting element ( 10 ) with a coating ( 15 ) applied externally to the connecting element ( 10 ), where the connecting element ( 10 ) is guided, in particular punched, through the first workpiece layer ( 1 ) by a ram ( 26 ), forming a cut surface ( 1 ), and penetrates at least partially into the second workpiece layer and is deformed by a die ( 21 ) such that it connects the at least two workpiece layers ( 1, 2 ) together in a form-fitting manner. The connecting element ( 10 ) is heated such that the coating ( 15 ) applied externally to the connecting element ( 10 ) melts and wets, in particular completely closes, the cut surface in the first workpiece layer ( 1 ); after a cooling process, the re-solidified coating ( 15 ) adheres to the cut surface of the first workpiece layer ( 1 ).

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a United States National Phase application of International Application PCT/EP2013/055430 filed Mar. 15, 2013 and claims the benefit of priority under 35 U.S.C. §119 of German patent application DE 10 2012 006 631.7 filed Mar. 31, 2012 and German patent application DE 10 2012 008 798.5 filed Apr. 19, 2012, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a method for joining at least two workpiece layers by introducing a connecting element having a coating which is applied to the outside of the connecting element, wherein the connecting element is introduced by means of a joining device at a joint and interconnects the at least two workpiece layers, in that by means of a stamp, the connecting element, from the direction of the first workpiece layer and under exertion of a stamping force, and while forming a cutting face in the first workpiece layer (1) or passing beside a cutting face of the first workpiece layer (1), is guided through, in particular punched through, at least the first workpiece layer and penetrates, at least in part, the second workpiece layer, and the connecting element, by means of a die plate bearing on the second workpiece layer, is deformed under exertion of the stamping force onto the connecting element, such that said connecting element, in a form-fitting manner, interconnects the at least two workpiece layers.

The invention furthermore relates to a connecting element and a joining device.

BACKGROUND OF THE INVENTION

The method of punch-riveting, for example, is generally known in the context of joining at least two workpiece layers. This method serves for the unreleasable force-fitting and form-fitting joining of the workpiece layers, using special punch rivets as connecting elements with which the workpiece layers, without pre-drilling, are punched through and joined in a combined punching and deforming operation. Here, the punch rivet serves as single-use cutting stamp and is itself also deformed in this case. The workpiece layers are initially positioned by means of a hold-down device over a die plate. Subsequently, the punch rivet, via a stamp, is impinged upon with a force and pressed into the join parts. Depending on the riveting method, the rivet shank, in the process, cuts through at least the uppermost workpiece layers and is then spread in the die plate. When using a hollow rivet, this takes place, in particular, without fully piercing the lowermost workpiece layer. In the case of punch-riveting of fiber-reinforced materials, such as, for example, glass-fiber-reinforced plastics or carbon-fiber-reinforced plastics, the fibers are cut through by the punch rivet, and delamination of fibers and the matrix material may arise in the peripheral zone of the cutting face. This may lead to fractures in the workpiece and a complete failure of the connection patch.

DE 10 2005 031 917 A1 discloses a joining method of the generic type for joining at least two sheet-like workpieces made of like materials or of different materials, in which method a punch-rivet connection is combined with an adhesive connection. Prior to the actual joining operation, an adhesive, which additionally adhesively bonds the workpiece layers to one another, is introduced into the joining zone between the workpiece layers. During the actual punch-riveting process, the adhesive flows radially outward from the joining zone, but does not reach the cutting faces and does not act against delamination.

US 2010/0088880 A1 discloses a connecting element which, within the rivet shank, has an adhesive which is pressed through passages of the rivet shank to the outer surface of the rivet shank only after the joining process has commenced, specifically during the punching operation. Prior to commencing the joining method, the connecting element is not coated on the outside.

SUMMARY OF THE INVENTION

The invention is based on the object of improving a joining method of the type mentioned at the outset, in particular with a view to largely or completely avoiding delamination in the region of the cutting faces of fiber-reinforced workpiece layers. Furthermore, a connecting element and a joining device for the implementation of the method according to the invention are to be provided.

This object is achieved according to the invention with a method having the method steps of the present invention, and a connecting element of the present invention, and a joining device of the present invention. Advantageous embodiments, which may be employed individually or in combination with one another, are the subject matter of the present invention.

On account of the connecting element being heated to such an extent that a coating of the connecting element melts and wets, in particularly completely seals, the cutting face in the first workpiece layer, and of the re-solidified coating, after a cooling process, adhering to the cutting face of the first workpiece layer, the cutting face is, in particular, completely sealed by the coating, and thereby delamination is prevented in the case of workpiece layers made of fiber-reinforced plastic.

One or more of the workpiece layers may be prepunched, such that a cutting face which has been created by prepunching is covered by means of the coating. However, the cutting face may also be produced during the method according to the invention by the connecting element itself. Through-punching connecting elements are particularly suitable for this purpose.

A through-punching connecting element is meant to be understood to be a connecting element which, during the joining operation, can punch through one or more workpiece layers, for example a punch rivet. However, a through-punching connecting element may also be guided through a prepunch which has been previously introduced into one or more workpiece layers without any problems, such that the connecting element itself does not punch a hole into the workpiece layers but substantially serves only for the connection of the workpiece layers. Advantageously, prior to commencing the method, at least one of the at least two workpiece layers, in particular the first workpiece layer, in the region of the joint, is a continuously closed surface without a prepunch. In this manner, the investment in processing is reduced.

Punch rivets, in particular semi-hollow punch rivets, are particularly suitable as through-punching connecting elements. The lowermost workpiece layer (thus the second workpiece layer in the case of two workpiece layers) is preferably not or not completely punched through by the punch rivet, such that a tight joint is created. Consequently, the generic term “through-punching connecting element” is also meant to include known connecting elements which do not completely punch through all workpiece layers, but one workpiece layer, in particular the second of two workpiece layers, is not punched through or only partially punched through. The method is advantageous also when solid punch rivets are used, in particular in the case where all workpiece layers are completely punched through.

By the connecting element having a through hole or a tapped hole, further functions may be integrated in the joint or linked to the joint.

The method is particularly advantageous for producing composites, of which the first workpiece layer is composed of a fiber-reinforced plastic and of which the second workpiece layer is composed of a metallic material, in particular a ferrous alloy, an aluminum alloy, or a magnesium alloy.

The coating of the connecting element preferably has an adhesive or a plastic, in particular a polymer material. For punch rivets, it is sufficient for their rivet shank to be coated.

Depending on the geometry and the material of the workpiece layers to be joined, it may be advantageous to preheat one or all workpiece layers, or the punch rivet, or the joining device, in an upstream method step.

Joining devices which comprise a die plate, a hold-down device and a stamp as tool elements, of which at least one tool element is heatable, are particularly suitable. Preferably, the stamp is heatable.

Thermal damage to the workpiece layers by way of excessive temperatures is avoided in that the joining device comprises a temperature-control device by means of which the temperature of the tool element, in particular of the stamp, can be adjusted and maintained at a constant level.

The present invention will be explained in more detail below on the basis of drawings, which show exemplary embodiments only. The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 is a view of two workpiece layers positioned between the hold-down device and the die plate, prior to the punch rivet being introduced;

FIG. 2 is a view showing the joint immediately after the punch rivet has been introduced into the joint;

FIG. 3 is a view showing the subsequent heating of the punch rivet and melting of the rivet coating;

FIG. 4 is a view showing the subsequent cooling of the joint with the coating adhering to the cutting faces; and

FIG. 5 is a view showing the completed joint immediately prior to the removal from the joining device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a first workpiece layer 1, and a second material layer 2 that is to be joined with the former, which layers have both been introduced into and been positioned in a joining device. The joining device, which is known per se, comprises a die plate 21, a hold-down device 24, and a stamp 26.

The first workpiece layer 1 is composed of a fiber-reinforced material, in the matrix 1 a of which a multiplicity of fibers 1 b have been introduced for the purpose of reinforcing the matrix material 1 a. The present fiber-reinforced material is a glass-fiber-reinforced plastic or carbon-fiber-reinforced plastic having oriented long fibers. However, in principle, it is possible for all known fiber-reinforced materials to be employed for the first workpiece layer 1. The present second material layer 2 is composed of a metallic material, in particular of a ferrous alloy, an aluminum alloy, or a magnesium alloy. However, in principle, it is also possible for other known non-fiber-reinforced or fiber-reinforced materials to be employed for the second workpiece layer 2.

The second material layer 2 bears on the die plate 21. The first material layer 1 bears in a two-dimensional manner on the second material layer 2. By means of the hold-down device 24, the two material layers 1, 2 are fixed in relation to one another in the joining device.

A connecting element in the form of a punch rivet 10 is positioned above the joint in the joining device. The punch rivet 10 is implemented as a semi-hollow punch rivet but, in a modified embodiment, could also be a solid rivet. The punch rivet 1 comprises a rivet head 11 and a rivet shank 12. The rivet shank 12 is provided with a coating 15 which is preferably composed of a polymer which is applied as an adhesive or a plastic to the rivet shank 12.

In FIG. 2, the joint is illustrated immediately after the combined punching and deforming operation in which the punch rivet 10 which, by means of the stamp 26, is impinged upon by a force is punched through the first workpiece layer 1 and is introduced into the second workpiece layer 2. The punch rivet 10 here serves as a single-use cutting stamp and, in this case, is also itself deformed in that it is spread on a molding in the die plate 21 by way of the counterforce which is applied by the die plate 21. In the present case, this takes place, in particular, without completely piercing the second workpiece layer 2. On account of the through-punching, an encircling, cylindrical cutting face is created in the first workpiece layer 1. In the region of the cutting face, the matrix material of the matrix 1 a and the fibers 1 b have been cut through. In the peripheral zone of the cutting face, the fiber ends of the fibers 1 b lie open and are not covered and protected by matrix material of the matrix 1 a. On account thereof, delamination of fibers 1 b and matrix material 1 a arises in the cutting face.

In a variant of the exemplary embodiment, the first workpiece layer 1 is prepunched already prior to commencing the method according to the invention, such that the cutting face in the first workpiece layer is not or only in part produced by the punch rivet 10.

In a next method step, which is shown in FIG. 3, the joint is heated by the stamp 26 in that the stamp 26 is heatable and is heated, in particular by electric power or induction. In an alternative joining device, the die plate 21 and/or the hold-down device 24 may also be heatable. On account of the introduced heat, the coating 15 of the rivet shank 12 of the punch rivet 10 melts and wets the cutting face in the first workpiece layer 1, such that the coating in particular completely seals the cutting face.

In a subsequent step, which is shown in FIG. 4, the joint is cooled in that the heat supply to the stamp 26 is switched off and, in particular, active cooling additionally takes place. To this end, the stamp 26 is already remote from the joint. The coating 15 now solidifies on the cutting face of the first workpiece layer 1 and thus covers the fiber ends of the fibers 1 b which, on account thereof, are protected against delamination.

In a last method step, the workpiece layers 1 and 2, which are joined together, are removed from the joining device (FIG. 5).

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the principles of the invention, it will be understood that the invention may be embodied otherwise without departing from such principles. 

1. A method for joining at least two workpiece layers, the method comprising: providing a connecting element having a coating which is applied to the outside of the connecting element, wherein the connecting element is introduced into the workpiece layers by means of a joining device at a joint; by means of a stamp, guiding the connecting element, from a direction of a first workpiece layer and under exertion of a stamping force, and while forming a cutting face in the first workpiece layer or passing beside a cutting face of the first workpiece layer, through at least the first workpiece layer and the connecting element penetrating, at least in part, the second workpiece layer, wherein the connecting element, by means of a die plate bearing on the second workpiece layer, is deformed under exertion of the stamping force onto the connecting element, such that said connecting element, in a form-fitting manner, interconnects the at least two workpiece layers; heating the connecting element, during the process, to such an extent that the coating melts and wets the cutting face in the first workpiece layer, wherein the coating re-solidifies after a cooling process to form a re-solidified coating, said re-solidified coating adhering to the cutting face of the first workpiece layer.
 2. A method as claimed in claim 1, wherein the connecting element is heated to such an extent that the coating melts and completely seals the cutting face in the first workpiece layer.
 3. A method as claimed in claim 1, wherein, prior to commencing the method, at least one of the at least two workpiece layers, in a region of the joint, has a continuously closed surface without a prepunch.
 4. A method as claimed in claim 1, wherein the connecting element is a through-punching connecting element, said connecting element being punched through at least the first workpiece layer.
 5. A method as claimed in claim 4, wherein the through-punching connecting element is a punch rivet.
 6. A method as claimed in claim 5, wherein the through-punching connecting element is a semi-hollow punch rivet or a solid punch rivet.
 7. A method as claimed in claim 1, wherein the connecting element has a through hole or a tapped hole.
 8. A method as claimed in claim 1, wherein at least one of the at least two workpiece layers is composed of fiber-reinforced plastic.
 9. A method as claimed in claim 1, wherein at least one of the at least two workpiece layers is composed of a metallic material.
 10. A method as claimed in claim 1, wherein the coating comprises an adhesive or a plastic.
 11. A method as claimed in claim 10, wherein the coating is composed of a thermoplastic.
 12. A connecting element comprising a coating for use in a method as claimed in claim
 1. 13. A joining device for use in a method as claimed in claim 1, the joining device comprising: a die plate; a hold-down device; and a stamp, said die plate, said hold-down device and said stamp defining tool elements, at least one of said tool elements being heatable.
 14. A joining device as claimed in claim 13, wherein the stamp is heatable.
 15. A joining device as claimed in claim 13, wherein the joining device comprises a temperature-control device by means of which a temperature of the at least one of said tool elements can be adjusted and maintained at a constant level.
 16. A method as claimed in claim 1, wherein at least one of the at least two workpiece layers is composed of a ferrous alloy, an aluminum alloy, or a magnesium alloy.
 17. A method as claimed in claim 1, wherein the coating comprises a polymer material. 